The invention relates to tracking techniques in satellite radio-navigation. Conventional radio navigation systems make use of simultaneous observations to all satellites in view over the local horizon. In order to unambiguously determine the four-dimensional position including the three geographical coordinates and the time, simultaneous “one-way range unambiguous” observations to at least four satellites in view plus the knowledge of the precise positions of these satellites is necessary.
These techniques are also referred to as “one-way range unambiguous” observation techniques and are described in the standard literature (e.g. “Global Positioning System: Theory and applications”, Vol. 1, American Institute of Aeronautics and Astronautics, 1996, edited by B. P. Parkinson and J. J. Spilker). The nature of the “one-way range unambiguous” observation makes it sensitive to any misalignment between the transmitter's and the receiver's clocks. As the transmitter's clock offset with respect to a reference time scale is accurately known, in a first approximation, the “one-way range unambiguous observation” depends only on the user-to-satellite range and on the receiver's-clock-offset with respect to the reference time scale. In conventional satellite radio-navigation the transmitter is on-board the satellite and the receiver is on-ground.
In addition, in order to reduce the noise of the “one-way range unambiguous” observations, it is known to additionally process “one-way range ambiguous” observations (e.g in professional applications) which is possible thanks to the much superior accuracy of the second type of observations (e.g. the “Carrier-Smoothing Method” discussed in the above-mentioned reference of Parkinson).
In satellite navigation, the most frequently used “one-way range” observations are the “pseudorange” and the “carrier phase” observations corresponding to a “one-way range unambiguous” observation and to a “one-way range ambiguous” observation respectively. Any “pseudorange” observable is visibly affected by multipath and interference if the “pseudoranges” to all the satellites in view over the local horizon are synchronously obtained (simultaneously) by a ground equipment with one common non-directional antenna.
If instead sequential “pseudorange” observations are obtained then all equipment resources can be focused on one single satellite, and an antenna with a much higher directivity can be used. The resulting sequential observations are much less affected by multipath and by interference, and preserved accurately the geometrical information as far as the stability of the antenna phase response is ensured.
However whereas for a set of synchronous “one-way range” observations all observations are affected by common errors and refer to the same receiver positions, for sequential “one-way range” observations errors are not common and the observations refer potentially to different positions (since the receiving station may move between the observations).
Since the positioning errors due to multipath and interference perturbations have been considered compatible with the past needs, tracking techniques (providing sequential “pseudoranges”) using antenna with higher directivity have been widely disregarded in satellite navigation in the past.